Influence of Electronic and Structural Effects on the Oxidative Behavior

Inorg. Chem. 2002, 41, 6673−6687

Influence of Electronic and Structural Effects on the Oxidative Behavior of Nickel Porphyrins Karl M. Kadish,*,† Min Lin,† Eric Van Caemelbecke,†,‡ Guido De Stefano,† Craig J. Medforth,*,§,| Daniel J. Nurco,§ Nora Y. Nelson,§ Be´ne´dicte Krattinger,§ Cinzia M. Muzzi,§ Laurent Jaquinod,§ Yang Xu,§ David C. Shyr,§ Kevin M. Smith,*,§ and John A. Shelnutt⊥,| Department of Chemistry, UniVersity of Houston, Texas 77204-5641, Department of Chemistry, Houston Baptist UniVersity, Texas 77074-3298, Department of Chemistry, UniVersity of California, DaVis, California 95616-5295, Biomolecular Materials and Interfaces Department, Sandia National Laboratories, Albuquerque, New Mexico 87185, and Department of Chemistry, UniVersity of Georgia, Athens, Georgia 30602-2556 Received January 24, 2002

With the aim of better understanding the electronic and structural factors which govern electron-transfer processes in porphyrins, the electrochemistry of 29 nickel(II) porphyrins has been examined in dichloromethane containing either 0.1 M tetra-n-butylammonium perchlorate (TBAP) or tetra-n-butylammonium hexafluorophosphate (TBAPF6) as supporting electrolyte. Half-wave potentials for the first oxidation and first reduction are only weakly dependent on the supporting electrolyte, but E1/2 for the second oxidation varies considerably with the type of supporting electrolyte. E1/2 values for the first reduction to give a porphyrin π-anion radical are effected in large part by the electronic properties of the porphyrin macrocycle substituents, while half-wave potentials for the first oxidation to give a π-cation radical are affected by the substituents as well as by nonplanar deformations of the porphyrin macrocycle. The potential difference between the first and second oxidations (∆|Ox2 − Ox1|) is highly variable among the 29 investigated compounds and ranges from 0 mV (two overlapped oxidations) to 460 mV depending on the macrocycle substituents and the anion of the supporting electrolyte. The magnitude of ∆|Ox2 − Ox1| is generally smaller for compounds with very electron-withdrawing substituents and when TBAP is used as the supporting electrolyte. This behavior is best explained in terms of differences in the binding strengths of anions from the supporting electrolyte (ClO4- or PF6-) to the doubly oxidized species. A closer analysis suggests two factors which are important in modulating ∆|Ox2 − Ox1| and thus the binding affinity of the anion to the porphyrin dication. One is the type of π-cation radical (a proxy for the charge distribution in the dication), and the other is the conformation of the porphyrin macrocycle (either planar or nonplanar). These findings imply that the redox behavior of porphyrins can be selectively tuned to display separate or overlapped oxidation processes.

Introduction Over the past three decades, nickel(II) porphyrins have been extensively studied as to their spectroscopic and redox properties and a detailed review of their electrochemistry in nonaqueous media has recently appeared in the literature.1 Nickel porphyrins can be oxidized in up to three one* To whom correspondence should be addressed. E-mail: [email protected] (K.M.K). † University of Houston. ‡ Houston Baptist University. § University of California. | Sandia National Laboratories. ⊥ University of New Mexico.

10.1021/ic0200702 CCC: $22.00 Published on Web 11/19/2002

© 2002 American Chemical Society

electron-transfer steps,2 depending upon the solvent, although the third oxidation is not always observed. They are also typically reduced in two one-electron-transfer steps, although some Ni(II) porphyrins with highly electron-withdrawing groups can be reduced by a total of three electrons.3 In most (1) Kadish, K. M.; Van Caemelbecke, E.; Royal, G. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: Boston, 2000; Vol. 8, p 1. (2) Kadish, K. M.; Caemelbecke, E. V.; Boulas, P.; D’Souza, F.; Vogel, E.; Kisters, M.; Medforth, C. J.; Smith, K. M. Inorg. Chem. 1993, 32, 4177. (3) Ozette, K.; Leduc, P.; Palacio, M.; Bartoli, J. F.; Barkigia, K. M.; Fajer, J.; Battioni, P.; Mansuy, D. J. Am. Chem. Soc. 1997, 119, 6442.

Inorganic Chemistry, Vol. 41, No. 25, 2002

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Kadish et al. cases, the initial oxidations and reductions involve the porphyrin π-system, but examples where the first electron is removed or added to the central metal ion to give a nickel(III) species4-6 or a nickel(I) species7-9 are well documented in the literature. The goal of the present study is to understand in more detail how the electronic properties of the peripheral substituents of a nickel porphyrin and the structural properties of the porphyrin macrocycle influence the oxidative behavior of the compound, and specifically how they are related to the electron-transfer mechanism. A previous electrochemical study of six nickel(II) porphyrins10 showed that the absolute potential separation between the first two oxidations, ∆|Ox2 - Ox1|, varied as a function of the type of supporting electrolyte and the substituents on the porphyrin macrocycle. In some cases, two one-electron oxidations were seen upon going from the neutral compound to the dication, and in other cases the two one-electron oxidations were overlapped. For example, depending upon the compound, ∆|Ox2 - Ox1| varied from 480 to 0 mV in dichloromethane solutions containing 0.1 M tetra-n-butylammonium perchlorate (TBAP).10 However, for the same compounds in dichloromethane solutions with tetra-n-butylammonium hexafluorophosphate (TBAPF6), ∆|Ox2 - Ox1| varied from 510 to 240 mV and none of the oxidations were overlapped. We wished to examine nickel porphyrins with a much larger range of substituents to determine the factors responsible for overlap of the oxidations. Such a study is discussed in this paper. The first section under Results and Discussion describes the electrochemical behavior of nickel porphyrins 1-29 (see Figure 1 and Table 1) in dichloromethane containing two different supporting electrolytes [0.1 M TBAP or 0.1 M TBAPF6]. The second section under Results and Discussion (Structural Studies) details the structures of the nickel porphyrins as determined using X-ray crystallography and molecular mechanics calculations. Porphyrins 1-29 can be divided into derivatives with T(aryl)P, T(alkyl)P, T(X)OEP, Br8T(aryl)P, and D(aryl)P substituent patterns (see Figure 1). These groups are shown to encompass significant variations in the type of nonplanar deformation of the porphyrin macrocycle induced by the peripheral substituents in addition to the expected electronic effects of the substituents. In the last section under Results and Discussion, the data obtained from the electrochemical and structural studies are used to analyze the dependence of the half-wave potentials on the electronic properties of the substituents, the structural properties of the porphyrin, and the type of supporting electrolyte. (4) Seth, J.; Palaniappan, V.; Bocian, D. F. Inorg. Chem. 1995, 34, 2201. (5) Connick, P. A.; Macor, K. A. Inorg. Chem. 1991, 30, 4654. (6) Connick, P. A.; Haller, K. J.; Macor, K. A. Inorg. Chem. 1993, 32, 3256. (7) Kadish, K. M.; Franzen, M. M.; Han, B. C.; Araullo-McAdams, C.; Sazou, D. J. Am. Chem. Soc. 1991, 113, 512. (8) Kadish, K. M.; Franzen, M. M.; Han, B. C.; Araullo-McAdams, C.; Sazou, D. Inorg. Chem. 1992, 31, 4399. (9) Lexa, D.; Momenteau, M.; Mispelter, J.; Save´ant, J.-M. Inorg. Chem. 1989, 28, 30. (10) Chang, D.; Malinski, T.; Ulman, A.; Kadish, K. M. Inorg. Chem. 1984, 23, 817.

6674 Inorganic Chemistry, Vol. 41, No. 25, 2002

Figure 1. Structures of the porphyrins investigated. Table 1. List of the Nickel Porphyrins Investigateda compd 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

group

macrocycle (P)

T(aryl)P

TPP T(4-Cl-P)P T(4-Et2N-P)P F20TPP T(alkyl)P T(tBu)P T(C3F7)P T(iPr)P T(Me)P T(Et)P T(Pr)P T(X)OEP T(Bz)OEP OET(3-Th)P OETPP OETNP F20OETPP TC6TPP Br8T(aryl)P Br8TPP Br8F20TPP D(aryl)P DPP F4DPP F8DPP F12DPP (OMe)20DPP OPT(3-Th)P F8DPP (meso) F20DPP F28DPP F36DPP T(4-NO2-P)OPP

meso substituents β substituents Ph 4-Cl-Ph 4-N(Et)2-Ph C6F5 tBu CF2CF2CF3 iPr Me Et Pr OCOPh 3-thienyl Ph NO2 C6F5 Ph Ph C6F5 Ph 4-F-Ph Ph 4-F-Ph 3,4,5-tri-OMe-Ph 3-thienyl 2,6-di-F-Ph C6F5 C6F5 C6F5 4-NO2-Ph

H H H H H H H H H H Et Et Et Et Et (CH2)4 Br Br Ph Ph 4-F-Ph 4-F-Ph 4-OMe-Ph Ph Ph Ph 4-F-Ph 3,5-di-F-Ph Ph

a Schematic representations of the different groups of macrocycles are shown in Figure 1.

Results and Discussion I. Electrochemical Studies. Reversible half-wave potentials for the reduction and oxidation of the 29 nickel(II) por-

OxidatiWe BehaWior of Nickel Porphyrins Table 2. Half-Wave Potentials (E1/2, V vs SCE) for Reduction or Oxidation of the Investigated Nickel Porphyrins in CH2Cl2 Containing 0.1 M TBAP or 0.1 M TBAPF6 TBAP compd TPP1,10 T(4-Cl-P)P1,10 T(4-Et2N-P)P1,10 F20TPP T(tBu)P T(C3F7)P T(iPr)P T(Me)P1,10 T(Et)P1,10 T(Pr)P1,10 T(Bz)OEP OET(3-Th)P OETPP67 OETNP F20OETPP TC6TPP Br8TPP Br8F20TPP DPP F4DPP F8DPP F12DPP (OMe)20DPP OPT(3-Th)P F8DPP (meso) F20DPP F28DPP F36DPP T(4-NO2-P)OPP

TBAPF6 - TBAP

TBAPF6

Red(1)

Ox(1)

Ox(2)

∆|Ox2 - Ox1|

Red(1)

Ox(1)

Ox(2)

∆|Ox2 - Ox1|

∆Red(1)

∆Ox(1)

∆Ox(2)

-1.28 -1.18 -1.36 -0.85 -1.48 -0.72 -1.44 -1.36 -1.39 -1.41 -1.23 -1.59 b -0.64a -1.19 -1.56 -0.80 -0.42 -1.37 -1.30 -1.26 -1.17 -1.38 -1.35 -1.19 -0.92 -0.80 d -1.13a

1.05 1.13 0.62 1.41 0.60 1.54 0.81 0.83 0.85 0.85 0.94 0.68 0.63 1.41 d 0.66 1.20 1.50 0.75 0.86 0.85 0.89 0.75 0.81 1.02 1.25 1.31 a 0.98

1.17 1.13 0.97 1.41 0.93 1.54 1.18 1.15 1.15 1.15 1.20 0.89 0.90 d d 0.98 1.20 1.50 0.83 0.86 0.85 0.89 0.87 0.81 1.16 1.25 1.31 d 0.98

0.12 0.00 0.35 0.00 0.33 0.00 0.37 0.32 0.30 0.30 0.26 0.21 0.27 e e 0.32 0.00 0.00 0.08 0.00 0.00 0.00 0.12 0.00 0.14 0.00 0.00 e 0.00

-1.31 -1.26 -1.43 -0.90 -1.50 -0.73 -1.46 -1.40 -1.45 -1.45 -1.24 -1.60 b -0.48a -1.19 b -0.87a -0.43 -1.42 -1.34 -1.30 -1.24 -1.38 b -1.26 -0.96 -0.82 -0.60 b

1.01 1.10 0.58 1.42 0.64 1.54 0.83 0.80 0.81 0.81 0.93 0.67 0.54 1.54 1.04 d 1.25 1.66 0.71 0.81 0.84 0.88 0.73 d 0.96 1.30 1.38 1.58 0.98

1.31 1.34 1.01 1.67 0.93 1.67 1.20 1.26 1.26 1.26 1.21 1.00 0.90 d 1.44 d 1.25 1.66 1.01 1.06 1.04 1.08 0.91 d 1.34 1.52 1.57 1.58 1.16

0.30 0.24 0.43 0.25 0.29 0.13 0.37 0.46 0.45 0.45 0.28 0.33 0.36 e 0.40 e 0.00 0.00 0.30 0.25 0.20 0.20 0.18 e 0.38 0.22 0.19 0.00 0.18

-0.03 -0.08 -0.07 -0.05 -0.02 -0.01 -0.02 -0.04 -0.06 -0.04 -0.01 -0.01 c c 0.00 c c -0.01 -0.05 -0.04 -0.04 -0.07 0.00 c -0.07 -0.04 -0.02 c c

-0.04 -0.03 -0.04 +0.01 +0.04 0.00 +0.02 -0.03 -0.04 -0.04 -0.01 -0.01 -0.09 +0.13 e e +0.05 +0.16 -0.04 -0.05 -0.01 -0.01 -0.02 e -0.06 +0.05 +0.07 e 0.00

+0.14 +0.21 +0.04 +0.26 0.00 +0.13 +0.02 +0.11 +0.11 +0.11 +0.01 +0.11 0.00 e e e +0.05 +0.16 +0.18 +0.20 +0.19 +0.19 +0.04 e +0.18 +0.27 +0.26 e 0.18

a E at 0.1 V/s. b E c pc 1/2 values were not measured. Not calculated because reductions have not been studied with both supporting electrolytes or because reductions are not reversible or are only reversible with one type of supporting electrolyte. d Value was not measured or process was not observed. e Could not be calculated because Ox(2) or Ox(1) values were not available.

Table 3. UV-Visible and ESR Data (λmax and × 10-4 m-1‚cm-1) of Neutral, Oxidized, and Reduced (P)NiII Derivatives in CH2Cl2 Containing 0.2 M TBAPF6 or 0.2M TBAP neutral

ESR

macrocycle (P)

B(0,0)

Q(1,0) (II)

Q(0,0) (I)

TPP10 T(4-Cl-P)P10 T(4-Et2N-P)P10 T(C3F7)P T(iPr)P T(Me)P10 T(Et)P T(Bz)OEP OET(3-Th)P TC6TPP DPP F4DPP F8DPP F12DPP (OMe)20DPP (F8DPP)(meso) F20DPP F28DPP

414 (19.1) 416 (19.8) 447 (16.8) 406 (9.54) 424 (7.2) 418 (18.9) 417 (18.6) 424 (28.9) 433 (14.3) 415 (7.0) 450 (17.0) 447 (16.9) 447 (18.8) 446 (14.3) 455 (14.0) 436 (20.6) 434 (18.5) 433 (19.0)

525 (3.7) 527 (2.5) 542 (3.0) 548 (0.6) 549 (0.6) 537 (1.2) 535 (1.1) 543 (2.2) 552 (0.9) 542 (0.4) 567 (1.7) 565 (1.3) 566 (1.7) 566 (1.3) 569 (1.5) 559 (1.8) 557 (1.6) 556 (1.9)

555 (sh)a 558 (sh)a 587 (2.6) 588 (1.7) 583 (0.1) 570 (sh)a 572 (sh)a 583 (0.7) 588 (0.7) 578 (0.5) 611 (1.8) 611 (1.4) 611 (1.8) 609 (1.3) 613 (1.8) 599 (2.2) 595 (2.4) 597 (2.7)

a

I/II

0.87 2.83 0.18 0.32 0.81 1.17 1.06 1.05 1.01 1.01 1.20 1.22 1.55 1.42

∆H

11.0 10.8 24.1 12.0 6.00 5.20

12.0

reduced g

oxidized

first

first

second

414 416 442

522 612 545

600 748 607

410 410 406

606 649 492

350 352 416

418 417

600 601

722 723

431 434

558 560

422 423

484 486 489 486 b 486 467 462

713 727 725 723 b 676 656 652

427 429 429 428 b 976 944 941

748 745 743 739 412 406 409 409

353 352 357 354 747 715 702 705

361 368 339 327

2.007 2.007 1.998 2.001 2.007 2.007

2.008

Shoulder peak. b Irreversible chemical reaction.

phyrins, obtained in dichloromethane containing 0.1 M TBAP or 0.1 M TBAPF6 as the supporting electrolyte, are presented in Table 2. Spectroscopic data were obtained for a selected group of the neutral, reduced, and oxidized forms of the electrochemically examined nickel porphyrins (Table 3) and confirmed that in all cases it was the porphyrin macrocycle and not the nickel atom that was oxidized or reduced under the conditions of our electrochemical experiments, i.e., in CH2Cl2 solutions containing 0.1 M TBAP or

0.1 M TBAPF6. For example, Figure S1 of the Supporting Information shows the EPR spectrum of the species generated upon oxidation of (F20DPP)Ni; the g value of 2.008 is consistent with the formation of a π-cation radical rather than a nickel(III) species. The potentials for the first reversible reduction of the 29 compounds at the porphyrin macrocycle to give a π-anion radical are listed as Red(1) in Table 2. The potentials vary significantly as a function of the peripheral substituents, with Inorganic Chemistry, Vol. 41, No. 25, 2002

6675

Kadish et al. Table 4. Assignments of Macrocycle Conformations and π-Cation Radicals for Nickel Porphyrins 1-29a

Figure 2. Effect of supporting electrolyte on electrochemical behavior of (DPP)Ni and (F20DPP)Ni. All cyclic voltammograms were recorded at a scan rate of 0.1 V/s.

E1/2 values ranging from -0.42 V in the case of (Br8F20TPP)Ni (18) to -1.60 V in the case of (OET(3-Th)P)Ni (12). The difference between the first reduction potentials in 0.1 M TBAP or 0.1 M TBAPF6, ∆Red(1), is also listed in Table 2. The values of ∆Red(1) are small, indicating that there is little effect of the supporting electrolyte on E1/2 for the first reduction. The range of E1/2 values for the first oxidation to give a Ni(II) π-cation radical is also large. These values are listed as Ox(1) in Table 2 and vary from +0.54 V in the case of (OETPP)Ni (13) to +1.66 V in the case of (Br8F20TPP)Ni (18). For several compounds, the effect of the supporting electrolyte on the first macrocycle oxidation, ∆Ox(1), is larger than when the macrocycle is reduced, although in a majority of cases the values of ∆Ox(1) are again small and of the same magnitude as ∆Red(1). This indicates only a small effect of the type of supporting electrolyte on E1/2 for the first oxidation. The relationship between the first macrocycle oxidation and first macrocycle reduction potentials and the electronic properties of the porphyrin substituents are discussed in more detail in Section III. The potential for the second macrocycle oxidation, Ox(2), which gives the Ni(II) π-dication, is significantly affected by the type of supporting electrolyte, in contrast to the first macrocycle oxidation potential which showed little dependence on the supporting electrolyte. Values of ∆Ox(2), the difference in the E1/2 values for the second oxidation of the same compound using 0.1 M TBAP or 0.1 M TBAPF6 as the supporting electrolyte, are given in Table 2. Changing from TBAP to TBAPF6 typically results in a positive shift in Ox(2) which can be as large as 270 mV. The absolute values of the separations between the first and second oxidations measured with the same supporting electrolyte, ∆|Ox2 - Ox1|, are summarized in Table 2 for each examined nickel porphyrin. The experimentally measured values of ∆|Ox2 - Ox1| range from 0 mV (i.e., two overlapping oxidations) to 460 mV depending upon the porphyrin macrocycle and the supporting electrolyte. The electronic and structural factors influencing ∆|Ox2 - Ox1| are also analyzed in more detail in section III. Two illustrations of how the supporting electrolyte affects the oxidation potentials are provided in Figure 2 for the compounds (DPP)Ni and (F20DPP)Ni. As seen in this figure, the first two oxidations of (DPP)Ni (19) are separated by


macrocycle (P)

π-cation radical

conformation

TPP T(4-Cl-P)P T(4-Et2N-P)P F20TPP T(tBu)P T(C3F7)P T(iPr)P T(Me)P T(Et)P T(Pr)P T(Bz)OEP OET(3-Th)P OETPP OETNP F20OETPP TC6TPP Br8TPP Br8F20TPP DPP F4DPP F8DPP F12DPP (OMe)20DPP OPT(3-Th)P F8DPP (meso) F20DPP F28DPP F36DPP T(4-NO2-P)OPP

a2u64-67 a2u a2u a1u a1u/a2u68 a1u a2u a2u a2u a2u a1u a1u a1u67 a1u a1u a1u a2u61 a1u a2u47 a2u a2u a2u a2u a2u a1u a1u a1u a1u a1u

planar16-18 planar (by analogy with 1) planar (by analogy with 1) planar (by analogy with 1) ruffled21,39 ruffled (X-ray and MM this work) ruffled21,23 planar19-21 planar21 planar21 ruffled (X-ray and MM this work) saddled (by analogy with 13, 15, 16) saddled14,34 mixed37,38 saddled (X-ray and MM this work) saddled14,34 saddled (by analogy with 18) saddled41 mixed40,46,47 mixed (MM this work) mixed (MM this work) mixed (MM this work) mixed (by analogy with other D(Ar)P) mixed (by analogy with other D(Ar)P) mixed (MM this work) mixed46 (MM this work) mixed (MM this work) mixed (MM this work) mixed (by analogy with other D(Ar)P)

a Conformations in bold were assigned on the basis of X-ray or molecular mechanics (MM) structures. Conformations in italics are those proposed by analogy with related compounds in the same group. Radical assignments in bold were taken from the literature. Other radical assignments are those proposed in the text.

300 mV in dichloromethane containing 0.1 M TBAPF6 but by only 80 mV in dichloromethane containing 0.1 M TBAP. The first oxidation shifts positively by 40 mV upon going from TBAPF6 to TBAP while the second oxidation shifts negatively by 180 mV, thus resulting in two partially overlapped processes with a ∆|Ox2 - Ox1| value of 80 mV. The two oxidations are more completely overlapped for the case of (F20DPP)Ni (26) in TBAP (∆|Ox2 - Ox1| ) 0.0 V) as compared to oxidation of the same compound in solutions of TBAPF6 where the separation between the two oxidations is 220 mV. II. Structural Studies. Previous studies1,11-13 have indicated that the substituents affect the redox behavior of the porphyrin ring via direct electronic effects and by nonplanar deformations of the macrocycle induced by crowding at the porphyrin periphery. To aid in understanding the influence of structural effects on the electrochemical behavior of porphyrins 1-29, the conformations of the porphyrins were analyzed using X-ray crystallography and molecular mechanics (MM) calculations. Table 4 gives an overview of the structural characteristics (planar, saddled or ruffled or mixed (saddled and/or ruffled) of nickel porphyrins 1-29, Table 5 provides more detailed structural data for some of (11) Ravikanth, M.; Chandrashekar, T. K. Struct. Bonding (Berlin) 1995, 82, 105. (12) Shelnutt, J. A.; Song, X.-Z.; Ma, J.-G.; Jia, S.-L.; Jentzen, W.; Medforth, C. J. Chem. Soc. ReV. 1998, 27, 31. (13) Senge, M. O. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: Boston, 2000; Vol. 1, p 239.

OxidatiWe BehaWior of Nickel Porphyrins Table 5. Selected X-ray Crystallographic Data for Nickel Porphyrins porphyrin “planar” (TPP)Ni (T(Me)P)Ni (OEP)Ni (triclinic A) (OEP)Ni (tetragonal) “saddled” (OETPP)Ni (F20OETPP)Ni (TC6TPP)Ni (Br8F20TPP)Ni, “ruffled” (T(C3F7)P)Ni (T(Bz)OEP)Ni (T(iPr)P)Ni (Br8T(CF3)P)Ni (T(Bu)OEP)Ni “mixed” (DPP)Ni (DPP)Ni (DPP)Ni

Ni-Na (Å)

φpyrb (deg)

∆Cβc (Å)

∆Cmd (Å)

∆24e (Å)

1.931 1.943 1.958 1.929

14.4 3.3 1.7 16.4

0.18 0.08 0.06 0.27

0.45 0.07 0.03 0.51

0.22 0.07 0.02 0.26

16 19,20 85 86

1.906 1.901 1.914 1.898

29.4 25.8 24.8 27.9

1.23 1.15 1.08 1.16

0.03 0.05 0.02 0.19

0.62 0.59 0.54 0.61

34 this work 34 79

1.886 1.885 1.896 1.88 1.873

27.1 25.7 22.8 32.0 31.2

0.33 0.31 0.27 0.38 0.37

0.86 0.79 0.74 1.06 1.05

0.42 0.39 0.36 0.50 0.46

this work this work 27 26 27

1.909 1.894 1.885

27.8 29.7 31

0.36 0.35 1.02

0.86 0.92 0.60

0.43 0.45 0.59

46 47 47

ref

a Average nickel-nitrogen distance. b Average angle between the least-squares plane of the 24 atoms of the porphyrin macrocycle and the least-squares planes of the pyrrole rings. c Average displacement of the β carbon atoms from the least-squares plane of the 24 atoms of the porphyrin ring. d Average displacement of the meso carbon atoms from the least-squares plane of the 24 atoms of the porphyrin ring. e Average displacement of the 24 atoms of the porphyrin macrocycle from the least-squares plane of the 24 atoms of the porphyrin ring.

Table 6. Selected Crystallographic Experimental Data for Compounds 6, 11, and 15a

chemical formula formula weight space group a (Å) b (Å) c (Å) β (deg) volume (Å3) Z λ (Å) T (K) µ (mm-1) Dcalcd (g cm-3) R (Fo2) (>2σ(I)) Rw (Fo2) (all data)

6

11

15

C32H8F28N4Ni 1039.13 C2/c (No. 15) 21.328(4) 16.957(3) 21.573(4) 118.82(3) 6836(2) 8 1.541 78 130(2) 2.607 2.019 0.0654 0.1715

(C64H60N4NiO8)‚(CH2Cl2) 1156.85 C2/c (No. 15) 22.839(12) 17.915(9) 16.124(8) 122.030(9) 5593(5) 4 0.710 73 90(2) 0.504 1.373 0.0993 0.2111

C60H40F20N4Ni‚4(H2O) 1327.73 C2 (No. 5) 25.945(5) 8.276(2) 14.357(3) 108.57(3) 2922.2(10) 2 0.710 73 130(2) 0.447 1.509 0.0630 0.1741

a R1 ) ∑||F - F ||/∑|F | and wR2 ) [∑[w(F 2 - F 2)2]/∑[w(F 2)2]]1/2; w ) 1/[σ2(F 2) + ((X)P)2 + (Y)P], where P ) (F 2 + 2F 2)/3. For 6, X ) o c o o c o o o c 0.092 600 and Y ) 58.846 199; for 11, X ) 0.060 500 and Y ) 204.242 783; for 15, X ) 0.083 800 and Y ) 3.272 800.

the nickel porphyrins, and Table 6 summarizes crystallographic data for the three nickel porphyrins characterized in this work. Table S1 of the Supporting Information summarizes the energies obtained for different conformations of the (D(Ar)P)Ni macrocycles 19-22 and 25-28 using MM calculations.14,15 X-ray and MM studies of (TPP)Ni (1)16-18 and (T(alkyl)P)Ni macrocycles with methyl (8), ethyl (9), or propyl (10) groups19-21 indicate that these porphyrins typically adopt (14) Shelnutt, J. A.; Medforth, C. J.; Berber, M. D.; Barkigia, K. M.; Smith, K. M. J. Am. Chem. Soc. 1991, 113, 4077. (15) Song, X.-Z.; Jaquinod, L.; Jentzen, W.; Nurco, D. J.; Jia, S.-L.; Khoury, R.; Ma, J.-G.; Medforth, C. J.; Smith, K. M.; Shelnutt, J. A. Inorg. Chem. 1998, 37, 2009. (16) Maclean, A. L.; Foran, G. J.; Kennedy, B. J.; Turner, P.; Hambley, T. W. Aust. J. Chem. 1996, 49, 1273. (17) Jentzen, W.; Unger, E.; Song, X. Z.; Jia, S. L.; TurowskaTyrk, I.; Schweitzer-Stenner, R.; Dreybrodt, W.; Scheidt, W. R.; Shelnutt, J. A. J. Phys. Chem. A 1997, 101, 5789. (18) Rush, T. S.; Kozlowski, P. M.; Piffat, C. A.; Kumble, R.; Zgierski, M. Z.; Spiro, T. G. J. Phys. Chem. B 2000, 104, 5020. (19) Ulman, A.; Gallucci, J.; Fisher, D.; Ibers, J. A. J. Am. Chem. Soc. 1980, 102, 6852.

planar and/or slightly nonplanar conformations. These uncrowded porphyrins are not intrinsically very nonplanar because they lack bulky substituents or appreciable crowding of the substituents. The structures of this group of porphyrins are characterized by long Ni-N bond distances (e.g., 1.943 Å in (T(Me)P)Ni and 1.929 Å in (TPP)Ni), small out-ofplane displacements of the meso and β carbon atoms, small angles of the pyrrole rings with respect to the least-squares plane of the porphyrin ring, and small average out-of-plane displacements of the atoms of the porphyrin core (∆24 values) (see Table 5). Some uncrowded nickel porphyrins have been shown to exist as mixtures of planar and slightly nonplanar conformations in solution,17,22 reflecting a tradeoff between the requirement of the porphyrin to maintain a planar (20) Pace, L. J.; Ulman, A.; Ibers, J. A. Inorg. Chem. 1982, 21, 199. (21) Jentzen, W.; Hobbs, J. D.; Simpson, M. C.; Taylor, K. K.; Ema, T.; Nelson, N. Y.; Medforth, C. J.; Smith, K. M.; Veyrat, M.; Mazzanti, M.; Ramasseul, R.; Marchon, J.-C.; Takeuchi, T.; Goddard, I. W. A.; Shelnutt, J. A. J. Am. Chem. Soc. 1995, 117, 11085. (22) Alden, R. G.; Crawford, B. A.; Doolen, R.; Ondrias, M. R.; Shelnutt, J. A. J. Am. Chem. Soc. 1989, 111, 2070.


6677

Kadish et al. π-system and the requirement of the nickel(II) atom for a short metal-nitrogen distance; the latter forces the macrocycle into a nonplanar conformation. Molecular mechanics (MM) calculations and experimental studies confirm the presence of small energy differences between planar and nonplanar structures of β-octaalkylporphyrins and mesotetraarylporphyrins.17,22 All of the uncrowded porphyrins used in this study (1-4 and 8-10) are classified as planar in Table 4, signifying that they adopt planar or slightly nonplanar structures. In the case of the tetraalkylporphyrins with larger substituents [e.g., (T(tBu)P)Ni (5), (T(C3F7)P)Ni (6), and (T(iPr)P)Ni (7)], the increased peripheral steric repulsion is expected to make the porphyrin macrocycles more nonplanar. The crystal structure23 and MM structure21 of (T(iPr)P)Ni are best described in terms of a moderately ruffled conformation. In the ruffled conformation, the pyrrole rings are twisted alternately clockwise or anticlockwise such that the meso positions are moved alternately above or below the least-squares plane of the porphyrin ring.24 The average outof-plane displacement of the meso carbon atoms is 0.74 Å, and the Ni-N distance is shortened to 1.896 Å, consistent with the core contraction that is known to occur with the ruffling deformation mode.15 The bulkier tert-butyl groups in (T(tBu)P)Ni (5) provide even greater steric repulsion and force the macrocycle into a strongly ruffled conformation, as evidenced by crystallographic data25 and by the results of MM calculations.21 Other examples of strongly ruffled macrocycles are seen for 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(trifluoromethyl)porphinatonickel(II) [(Br8T(CF3)P)Ni]26 and 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetrabutylporphinatonickel(II) [(T(Bu)OEP)Ni],27 where the displacements of the meso carbon atoms (approximately 1.05 Å) are much larger than those seen for (T(iPr)P)Ni (Table 5). The crystal structure of (T(C3F7)P)Ni had not been determined prior to this work, and on the basis of data available for other metal complexes having this macrocycle, the structural assignment was not immediately evident. The crystal structures of the free base,28 the zinc pyridine complex,29 and the iron(II) bispyridine complex30 all show porphyrin macrocycles that are moderately ruffled, with average deviations of the meso carbon atoms from the leastsquares plane of the porphyrin plane (free base) or the four nitrogen atoms (Fe or Zn complex) of 0.29, 0.41, and 0.62 (23) Ema, T.; Senge, M. O.; Nelson, N. Y.; Ogoshi, H.; Smith, K. M. Angew. Chem., Int. Ed. Engl. 1994, 33, 1879. (24) Scheidt, W. R.; Lee, Y. J. Struct. Bonding (Berlin) 1987, 64, 1. (25) Drain, C. M.; Gentemann, S.; Roberts, J. A.; Nelson, N. Y.; Medforth, C. J.; Jia, S. L.; Simpson, M. C.; Smith, K. M.; Fajer, J.; Shelnutt, J. A.; Holten, D. J. Am. Chem. Soc. 1998, 120, 3781. (26) Nelson, N. Y.; Medforth, C. J.; Nurco, D. J.; Jia, S.-L.; Shelnutt, J. A.; Smith, K. M. Chem. Commun. 1999, 2071. (27) Senge, M. O.; Renner, M. W.; Kalisch, W. W.; Fajer, J. J. Chem. Soc., Dalton Trans. 2000, 381. (28) DiMagno, S. G.; Williams, R. A.; Therien, M. J. J. Org. Chem. 1994, 59, 6943. (29) Goll, J. G.; Moore, K. T.; Ghosh, A.; Therien, M. J. J. Am. Chem. Soc. 1996, 118, 8344. (30) Moore, K. T.; Fletcher, J. T.; Therien, M. J. J. Am. Chem. Soc. 1999, 121, 5196.


Figure 3. Side views of crystal structures of (a) (T(C3F7)P)Ni, (b) (T(Bz)OEP)Ni, and (c) (F20OETPP)Ni illustrating the nonplanar conformations of the porphyrin macrocycles and the orientations of the peripheral substituents. Hydrogen atoms have been omitted for clarity.

Å, respectively. In contrast, the crystal structure of the cobalt(II) complex showed a very nonplanar structure which was predominantly saddle distorted.31 In the saddle conformation, the pyrrole rings are tilted alternately up or down with respect to the least-squares plane of the porphyrin ring.24 The crystal structure of (T(C3F7)P)Ni is shown in Figure 3a. Crystallographic data for (T(C3F7)P)Ni are provided in Table 6, and selected structural parameters are summarized in Table 5. The crystal structure shows that (T(C3F7)P)Ni adopts a moderately ruffled structure, with the meso carbon atoms being displaced out of the least-squares plane of the 24 atoms of the porphyrin core by an average of 0.86 Å, the pyrrole rings being twisted by approximately 27° with respect to the porphyrin plane, and a short Ni-N distance of 1.886 Å (Table 5). The degree of ruffling is larger than that seen for the zinc(pyridine) complex29 and the iron(II)(pyridine)2 complex,30 in agreement with the known tendency of small metal atoms to induce additional nonplanar deformation.32 The minimum structure obtained from molecular mechanics (MM) calculations is in close agreement with the crystal structure of (T(C3F7)P)Ni. On the basis of the X-ray and MM data, the structures of (T(Bu)P)Ni (T(C3F7)P)Ni, and (T(Bu)P)(31) DiMagno, S. G.; Wertsching, A. K.; Ross, C. R. J. Am. Chem. Soc. 1995, 117, 8279. (32) Sparks, L. D.; Medforth, C. J.; Park, M.-S.; Chamberlain, J. R.; Ondrias, M. R.; Senge, M. O.; Smith, K. M.; Shelnutt, J. A. J. Am. Chem. Soc. 1993, 115, 581.

OxidatiWe BehaWior of Nickel Porphyrins

Ni are designated as ruffled in Table 4. However, we note that the T(C3F7)P macrocycle may exhibit some conformational flexibility, as suggested by a recent theoretical study.33 The other three classes of porphyrins in Table 1, T(X)OEP (11-16), Br8T(aryl)P (17,18), and D(aryl)P (19-29), are all expected to adopt very nonplanar structures to minimize steric crowding of the peripheral substituents.13 Compared to the nominally planar porphyrins, these macrocycles are characterized by short Ni-N bond distances (e1.91 Å), large out-of-plane displacements of the meso or β carbon atoms, large tilt or twist angles for the pyrrole rings (25-30°), and large average out-of-plane displacements for the porphyrin atoms (0.5-0.6 Å) (Table 5). Three of the six electrochemically investigated T(X)OEP complexes in Table 1 (13, 14, and 16) have been structurally characterized. (OETPP)Ni (13) and (TC6TPP)Ni (16) adopt saddle structures in the crystalline state.34 Saddle conformations have also been observed for a range of other OETPP complexes.32,35,36 In the case of (OETPP)Ni, the pyrrole rings are tilted by approximately 29° with respect to the leastsquares plane of the porphyrin ring, and the β carbon atoms are on average 1.24 Å out of the porphyrin plane. (TC6TPP)Ni is slightly more planar than (OETPP)Ni (Table 5). Saddle structures are also obtained from MM calculations on OETPP(Ni) and TC6TPP(Ni).14 An X-ray structure of (OETNP)Ni (14) also shows a saddle conformation.37 However, MM calculations38 indicate that (OETNP)Ni is conformationally flexible and that saddle, ruffle, or hybrid saddle/ruffle structures have similar energies. Accordingly, (OETNP)Ni is designated as a mixed structure while (OETPP)Ni and (TC6TPP)Ni are assigned as saddled structures in Table 4. The structures of (T(Bz)OEP)Ni (11) and (F20OETPP)Ni (15) were determined by X-ray crystallography (Table 6). As shown in Figure 3b, (T(Bz)OEP)Ni adopts a ruffled structure with only minor differences in the average out-ofplane displacement of the meso carbon atoms, the twist angle of the pyrrole ring, and the Ni-N distance compared to (T(C3F7)P)Ni (6) (Table 5). The ruffled structures seen for (T(Bz)OEP)Ni, (T(C3F7)P)Ni, and (T(tBu)P)Ni39 as well as for dodecaalkylporphyrins27,40 support the earlier suggestion40 that the asymmetry of an sp3 meso substituent favors a ruffled conformation. One significant difference between the crystal structures of (T(Bz)OEP)Ni (11) and (T(C3F7)P)Ni (6) is the orientation of the meso substituents. In both (T(Bz)OEP)Ni and the dodecaalkylporphyrins,27,40 the meso substituents adopt a (33) Wondimagegn, T.; Ghosh, A. J. Phys. Chem. A 2000, 104, 4606. (34) Barkigia, K. M.; Renner, M. W.; Furenlid, L. R.; Medforth, C. J.; Smith, K. M.; Fajer, J. J. Am. Chem. Soc. 1993, 115, 3627. (35) Barkigia, K. M.; Berber, M. D.; Fajer, J.; Medforth, C. J.; Renner, M. W.; Smith, K. M. J. Am. Chem. Soc. 1990, 112, 8851. (36) Regev, A.; Galili, T.; Medforth, C. J.; Smith, K. M.; Barkigia, K. M.; Fajer, J.; Levanon, H. J. Phys. Chem. 1994, 98, 2520. (37) Senge, M. O. J. Chem. Soc., Dalton Trans. 1993, 3539. (38) Hobbs, J. D.; Majumder, S. A.; Luo, L.; Sickelsmith, G. A.; Quirke, J. M. E.; Medforth, C. J.; Smith, K. M.; Shelnutt, J. A. J. Am. Chem. Soc. 1994, 116, 3261. (39) Barkigia, K. M.; Fajer, J. Personal communication. (40) Medforth, C. J.; Senge, M. O.; Smith, K. M.; Sparks, L. D.; Shelnutt, J. A. J. Am. Chem. Soc. 1992, 114, 9859.

pseudoaxial orientation in which the second carbon atom of the substituent (or the CdO group in (T(Bz)OEP)Ni)) is displaced toward the same face of the molecule as the meso carbon substituent (i.e., away from the plane of the porphyrin ring). In contrast, the perfluoroalkyl groups in (T(C3F7)P)Ni adopt pseudoequatorial orientations in which the second carbon atom of the substituent chain is folded back down toward the plane of the porphyrin ring. The crystal structure of (F20OETPP)Ni is illustrated in Figure 3c. The structure reveals that the porphyrin ring adopts a saddle conformation, with a large average out-of-plane displacement of the β carbon atoms (1.15 Å) and an average tilting of the pyrrole rings of approximately 26°. The structural parameters for (F20OETPP)Ni are very similar to those for (OETPP)Ni.34 MM calculations for (T(Bz)OEP)Ni and (F20OETPP)Ni give minimum energy structures which are similar to those determined crystallographically. (OET(3-Th)P)Ni (12) is also assigned as a saddle structure by analogy with (OETPP)Ni and (F20OETPP)Ni. (Br8F20TPP)Ni has been shown by X-ray crystallography to adopt a saddle conformation.41 Several other complexes of Br8F20TPP have also been shown to adopt saddle structures: (Br8F20TPP)Cu,41 (Br8F20TPP)Zn,42 and (Br8F20TPP)H2.43 The crystal structure of (Br8TPP)Ni has not been determined, although both (Br8TPP)H244 and (Br8TPP)Zn(PrCN)245 adopt structures which are saddle distorted. Given the preponderance of data showing that Br8T(Ar)P macrocycles favor saddle conformations, (Br8TPP)Ni is assigned a saddle structure in Table 4. The D(aryl)P derivatives are known to possess a greater degree of conformational flexibility than other dodecasubstituted porphyrins.46 For example, no fewer than three crystalline forms of the parent compound (DPP)Ni have been characterized.46,47 Crystallographic studies of (DPP)Ni and (F20DPP)Ni indicate that nickel complexes of the D(aryl)P derivatives can crystallize in saddle, ruffle, or hybrid saddle/ ruffle structures.40,46,47 Molecular modeling studies are consistent with the crystallographic results as they demonstrate that ruffle and saddle conformations are the favored structures of the fluorinated D(aryl)Ps 19-22 and 26-28. Moreover, these structures are separated by less than 4 kcal mol-1 (see Table S1 of the Supporting Information). On the basis of these data, all of the D(aryl)Ps were assigned as mixed structures in Table 4. In summary, X-ray crystallography and molecular mechanics calculations can be used to divide the 29 nickel (41) Henling, L. M.; Schaefer, W. P.; Hodge, J. S.; Hughes, M. E.; Gray, H. B.; Lyons, J. E.; Ellis, P. E. Acta Crystallogr., Sect. C 1993, C49, 1743. (42) Marsh, R. E.; Schaefer, W. P.; Hodge, J. A.; Hughes, M. E.; Gray, H. B. Acta Crystallogr., Sect. C 1993, C49, 1339. (43) Birnbaum, E. R.; Hodge, J. A.; Grinstaff, M. W.; Schaefer, W. P.; Henling, L.; Labinger, J. A.; Bercaw, J. E.; Gray, H. B. Inorg. Chem. 1995, 34, 3625. (44) Bhyrappa, P.; Krishnan, V.; Nethaji, M. Chem. Lett. 1993, 869. (45) Bhyrappa, P.; Krishnan, V.; Nethaji, M. J. Chem Soc., Dalton Trans. 1993, 1901. (46) Nurco, D. J.; Medforth, C. J.; Forsyth, T. P.; Olmstead, M. M.; Smith, K. M. J. Am. Chem. Soc. 1996, 118, 10918. (47) Barkigia, K. M.; Nurco, D. J.; Renner, M. W.; Melamed, D.; Smith, K. M.; Fajer, J. J. Phys. Chem. B 1998, 102, 322.


6679

Kadish et al. porphyrins into four basic structural groups: planar or nearly planar (1-4, 8-10), saddled (12, 13, 15-18), ruffled (57, 11), and mixed nonplanar (14, 19-29). On the basis of the observed out-of-plane distortions of the β carbon atoms, the saddled porphyrins 12, 13, and 15-18 are seen to have broadly similar out-of-plane deformations, except for (TC6TPP)Ni 16 which is slightly more planar than the other porphyrins. In the case of the porphyrins with ruffled macrocycles, 6, 7, and 11 display similar out-of-plane displacements of the meso carbon atoms (0.80 ( 0.06 Å) but (T(Bu)P)Ni (5) is significantly more nonplanar. III. Electronic and Structural Effects on Half-Wave Potentials. Previous electrochemical studies of porphyrins have demonstrated that in some cases it is possible to observe a linear relationship between E1/2 for oxidation or reduction of the macrocycle and the substituent Hammett σ constants, whereas in other cases the electrochemical potentials cannot be reliably predicted based on the electronic properties of the substituents.1 Early studies of electrochemical substituent effects in parasubstituted T(Ar)Ps48-53 used the phenyl ring as the baseline aromatic system. It was shown that the oxidation or reduction potentials varied linearly with the Hammett σ parameters of the para substituents. Studies of T(Ar)P complexes with substituents at the β-pyrrole positions, where the tetraphenylporphyrin moiety was assumed to be the aromatic system being substituted, produced more varied results.54,55 These papers also raised the question of which type of Hammett σ parameter (σm, σp, or the resonance parameters σp+ or σp-) should be used in the correlations. In the case of 2-substituted tetraphenylporphyrins,54,55 a linear free energy relationship was observed with the reduction potential when the resonance parameter σp- was used in the correlations. However, the best agreement between the oxidation potential and Hammett σ parameters was obtained using σp.54 In addition, progressive cyanation at the pyrrole positions of tetraphenylporphyrin resulted in additive shifts in the reduction potentials but nonadditive shifts in the oxidation potentials.54 These results were interpreted in terms of oxidation taking place at the nitrogen lone pair but reduction occurring at the porphyrin π-system. Subsequent studies also revealed marked nonadditivity in the oxidation potentials but not the reduction potentials upon progressive bromination of the pyrrole positions of T(Ar)Ps.55-57 These and more recent studies have led to the view that it is nonplanar (48) Kadish, K. M.; Morrison, M. M. J. Am. Chem. Soc. 1976, 98, 3326. (49) Walker, F. A.; Beroiz, D.; Kadish, K. M. J. Am. Chem. Soc. 1976, 98, 3484. (50) Kadish, K. M.; Morrison, M. M.; Constant, L. A.; Dickens, L.; Davis, D. G. J. Am. Chem. Soc. 1976, 98, 8387. (51) Kadish, K. M.; Morrison, M. M. Inorg. Chem. 1976, 15, 980. (52) Kadish, K. M.; Morrison, M. M. Bioinorg. Chem. 1977, 7, 107. (53) Kadish, K. M.; Morrison, M. M. Bioelectrochem. Bioenerg. 1977, 3, 480. (54) Giraudeau, A.; Callot, H. J.; Gross, M. Inorg. Chem. 1979, 18, 201. (55) Giraudeau, A.; Callot, H. J.; Jordan, J.; Ezhar, I.; Gross, M. J. Am. Chem. Soc. 1979, 101, 3857. (56) Ochsenbein, P.; Akoyougou, K.; Mandon, D.; Fischer, J.; Weiss, R.; Austin, R. N.; Jayarag, K.; Gold, A.; Terner, J.; Fajer, J. J. Angew. Chem., Int. Ed. Engl. 1994, 33, 348. (57) D’Souza, F.; Zandler, M. E.; Tagliatesta, P.; Ou, Z.; Shao, J.; Van Caemelbecke, E.; Kadish, K. M. Inorg. Chem. 1998, 37, 4567.


deformations of the porphyrin macrocycle induced by peripheral crowding which lead to the observed nonadditivity in the oxidation potentials.1,11-13 Theoretical studies11-13 are consistent with this view as they show that nonplanarity destabilizes the HOMO more than the LUMO. The relationship between the half-wave potentials and the electronic properties of the substituents was analyzed for nickel porphyrins 1-29. Given the diverse nature of the porphyrins in the present study, porphine (porphyrin) was used as the baseline aromatic system when examining the effect of the substituent electronic properties on the macrocycle oxidation and reduction potentials. The Hammett σ parameters for the peripheral substituents were taken from the literature58 and are listed in Table S2 of the Supporting Information. The total electronic effect of the substituents (∑σ) was estimated by adding the Hammett σ parameters for the substituents on the porphyrin ring. All of the commonly used Hammett σ parameters were used in attempting to find the best correlations: σm, σp, and the resonance parameters σp+ (for the oxidized species) and σp(for the reduced species). Equal weight was given to the substituents at the meso and β positions when calculating ∑σ. For example, ∑σp for (F20DPP)Ni was calculated by summing the Hammett σ parameters for the four C6F5 meso substituents (4 × 0.27) and for the eight C6H5 β substituents (8 × -0.01) to give a ∑σp value of 1.00. Calculated sums of the substituent parameters for each of the porphyrins investigated, using either σm, σp, σp+, or σp-, are given in Table S3 of the Supporting Information. Figure 4 illustrates the plots obtained for the first oxidation and first reduction versus ∑σ. Figure 4a shows the plot obtained for Ox(1) versus ∑σp+ and Figure 4b the plot obtained for Red(1) versus ∑σp-, both using TBAP as the supporting electrolyte. The correlation coefficient for the first macrocycle reduction process is poor for ∑σm (r ) 0.787 with 22 data points) but improves for ∑σp (r ) 0.933 with 22 data points) and is best for ∑σp- (r ) 0.940 with 21 data points). A similar trend is seen when TBAPF6 is the supporting electrolyte (see Table S4). The finding that the correlation coefficients improve with an increase in the resonance contribution to ∑σ agrees with the results obtained from earlier studies of porphyrins containing substituents which were directly attached to the porphyrin ring.54,55,59 Additional plots using porphyrins categorized according to whether they had planar or nonplanar conformations did not consistently improve the correlation coefficients obtained with the reduction potentials. Full details of the different correlations are given in Table S4 of the Supporting Information. A poorer correlation (r ) 0.918 for ∑σp-) was obtained for reduction of porphyrins 1-3 and 8-10, which have nominally planar structures. A somewhat better correlation (r ) 0.989 for ∑σp-) was obtained for porphyrins 12, 13, and 15-18, which have saddle structures with similar degrees of nonplanar deformation (out-of-plane distortions of approximately 1.2 Å for the β carbon atoms). (58) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165. (59) Wu, G.-Z.; Leung, H.-K.; Gan, W.-X. Tetrahedron 1990, 46, 3233.


Figure 4. Plots of (a) E1/2 for oxidation versus ∑σp+ and (b) E1/2 for reduction versus ∑σp- for nickel porphyrins. Values of E1/2 were measured in CH2Cl2 containing 0.1 M TBAP.

Poor correlation coefficients (r e 0.796) were obtained in the case of the first macrocycle oxidation Ox(1) irrespective of the Hammett parameters used in the plots. Differentiating the porphyrins into planar and nonplanar systems did improve the correlation coefficients, although the best agreement between E1/2 and ∑σ was not necessarily obtained with the resonance Hammett σ parameters. For example, in the case of the planar porphyrins 1-3 and 8-10, the correlation coefficient was 0.978 with ∑σp but only 0.905 for ∑σp+. For porphyrins 12, 13, and 15-18, which have saddle structures with similar amounts of nonplanar distortion, the correlation coefficients were 0.992 for ∑σp and 0.968 for ∑σp+. We note that in the case of 2-substituted tetraphenylporphyrins54,55 the oxidation potentials also correlated better with σp whereas the reduction potentials were more closely correlated with the resonance parameter σp-. E1/2 for the first reduction of the nickel porphyrins is a reasonable indicator of the electronic effect of the peripheral substituents, as evidenced by the approximately linear correlation obtained between E1/2 and ∑σp- for the large set of nickel porphyrins used in this study (Figure 4b). The data also indicate that a straightforward model involving equal weighting of the meso and β substituents adequately describes the electronic effect of the substituents. In the case of macrocycle oxidation, the electronic effect of the substituents is not the only factor contributing to the observed halfwave potentials, as demonstrated by the poor correlation in Figure 4a. The structural effects of the substituents (i.e., nonplanar deformations of the porphyrin macrocycle) must also be taken into account, and only for porphyrins with similar conformations (e.g., planar or saddle) are approximately linear correlations obtained with E1/2 for oxida-

tion. Overall, the plots obtained with a large number of nickel porphyrins support the prevailing view1,11-13 that structural (nonplanarity) effects are more significant in determining the half-wave potential for the first macrocycle oxidation to give the π-cation radical than for the first macrocycle reduction to give the π-anion radical. The second macrocycle oxidation of nickel porphyrins 1-29 to give the porphyrin dication shows a more significant dependence on the type of supporting electrolyte, as shown by the larger values of ∆Ox(2) in Table 2. The difference in half-wave potentials between the first and second oxidations, ∆|Ox2 - Ox1|, also varies considerably as a function of the porphyrin substituents and the type of supporting electrolyte (Table 2), and in some cases the two oxidations overlap. Two prior studies have examined the mechanism by which the relatively uncommon process of overlap of the oxidation reactions in porphyrins might occur.60,61 A disproportionation reaction of the π-cation radical was proposed to account for the overlapped oxidations in the very nonplanar porphyrins (Me8F20TPP)Zn, (Br8F20TPP)Zn, and (Cl8F20TPP)Zn.60 Specifically, saddling of the macrocycle in these highly substituted porphyrins was suggested to result in a1u π-cation radicals which were unstable and tended to disproportionate. In a more recent study, it was speculated that the instability of the π-cation radicals was related to the near degeneracy of the a1u and a2u HOMOs.61 It was proposed that a more pronounced pseudo-Jahn-Teller distortion in the dication versus the π-cation radical might favor the disproportionation reaction. The electrochemical and structural data for nickel porphyrins 1-29 was used to investigate the factors controlling ∆|Ox2 - Ox1|. Figure 5 shows plots of ∆|Ox2 - Ox1| versus E1/2 for the first macrocycle reduction (which was shown in Figure 4b to be an approximate measure of the electronic effect of the peripheral substituents). The data in Figure 5a are for oxidation potentials in CH2Cl2 containing TBAP as supporting electrolyte and in Figure 5b are for oxidation potentials in CH2Cl2 containing TBAPF6. Neither plot includes all of the 29 compounds, since only reversible electrode reactions were analyzed and a reversible reaction was not always observed. However, more than 20 sets of potentials are used in each plot. As shown in Figure 5b, only 2 of the 23 Ni(II) derivatives, (Br8F20TPP)Ni (18) and (F36DPP)Ni (28), exhibit a ∆|Ox2 - Ox1| of 0.00 V in CH2Cl2 containing TBAPF6 (compound 17 also has a 0.00 V gap but is not utilized in the plot because the first reduction of this porphyrin is not reversible). This contrasts with the results obtained in CH2Cl2 containing TBAP shown in Figure 5a, where 11 of the 23 investigated compounds (2, 4, 6, 18, 20, 21, 22, 24, 26, and 27) are characterized by overlapped oxidations with ∆|Ox2 - Ox1| ) 0.00 V. The general trend seen in Figure 5 is that the nickel porphyrins with more positive reduction potentials (i.e., more electron-deficient macrocycles) or with TBAP as the elec(60) Hodge, J. A.; Hill, M. G.; Gray, H. B. Inorg. Chem. 1995, 34, 809. (61) Ghosh, A.; Halvorsen, I.; Nilsen, H. J.; Steene, E.; Wondimagegn, T.; Lie, R.; van Caemelbecke, E.; Guo, N.; Ou, Z.; Kadish, K. M. J. Phys. Chem. B 2001, 105, 8120.


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Kadish et al. Scheme 1

Figure 5. Plots of ∆|Ox2 - Ox1| versus E1/2 for reduction in (a) dichloromethane containing 0.1 M TBAP and (b) dichloromethane containing 0.1M TBAPF6.

trolyte exhibit smaller values of ∆|Ox2 - Ox1|. This is consistent with the idea1 that the overlap of the macrocyclecentered oxidation potentials in the nickel porphyrins is in reality related to binding of the supporting electrolyte to the dication. Anion binding effectively stabilizes the dication with respect to the π-cation radical, as shown in the mechanism illustrated in Scheme 1 (where X ) ClO4- or PF6- from the supporting electrolyte). The ability of the anion from the supporting electrolyte to bind to oxidized porphyrins and stabilize higher oxidation states is well-known in the literature.62,63 In the presence of a weakly coordinating anion such as PF6-, the oxidation of (P)Ni to [(P)Ni]2+ will occur in two


separated one-electron-transfer steps labeled as E1 and E2 in Scheme 1. The potential E2 will clearly be more positive than potential E1. However, as the binding strength of [(P)Ni]+ and [(P)Ni]2+ with the anion of the supporting electrolyte increases, both oxidation potentials should be shifted in a negative direction, with the magnitude of the shift in E1/2 depending upon the formation constants of anion binding as shown by eqs 1-3. The magnitude of K1 is not large even in the case of ClO4-, since there is very little positive shift in E1/2 for the first oxidation upon going from PF6- to ClO4- as the anion of the electrolyte. This is apparent from the small values of ∆Ox(1) seen in Table 2. In contrast, the much larger values for ∆Ox(2) in Table 2 are indicative of much stronger binding of ClO4- to [(P)Ni]2+ and [(P)NiX]+. This results in a negative shift of the second oxidation potential leading to a smaller value of ∆|Ox2 - Ox1|. For example, for (F20DPP)Ni (26) in Figure 2 and Table 2 the first oxidation shifts negatively by 50 mV upon going from TBAPF6 to TBAP while the second shifts negatively by 270 mV. This is consistent with a much larger binding constant (K2K3) for coordination of the anion to [(P)Ni]2+ than for anion binding to [(P)Ni]+ (K1). The stabilization of the second oxidation product by ClO4- appears to be so strong in the case of some compounds (2, 4, 6, 17, 18, 20, 21, 22, 24, 26, and 27) that an immediate second oxidation occurs upon formation of the monocation, thus giving what appears to be an overall two-electron transfer, i.e., the conversion of (P)Ni to (P)NiX2 after the abstraction of two electrons. In this regard, it should be noted that the formal potentials given by E1 and E2 in Scheme 1 do not change when anion binding occurs but the actual observed reaction is easier (occurs at more negative potentials) due to the fact that the product is removed from solution by coordination. Interestingly, there is considerable scatter in the data in Figure 5 implying that factors other than simply the electronwithdrawing or electron-donating ability of the substituents are involved in determining the gap between the oxidation potentials. To better understand these factors, the variation (62) Seely, G. R.; Gust, D.; Moore, T. A.; Moore, A. L. J. Phys. Chem. 1994, 98, 10659. (63) Truxillo, L. A.; Davis, D. G. Anal. Chem. 1975, 47, 2260.


in ∆|Ox2 - Ox1| with Red(1) was analyzed further using two available factors: (a) the conformation of the porphyrin macrocycle (planar or nonplanar, as discussed earlier and summarized in Table 4) and (b) the type of radical (a1u or a2u) formed when the macrocycle was oxidized. The assignments proposed for the π-cation radicals of the nickel porphyrins (Table 4) were based on a limited number of earlier assignments, optical and ESR data for the nickel porphyrins (Tables 3 and 4), and the known abilities of electron-withdrawing or electron-donating substituents at the meso or β positions of the macrocycle to stabilize or destabilize a1u or a2u type radicals. T(aryl)P and T(alkyl)P porphyrins are typically expected to form a2u π-cation radicals upon oxidation.64-67 Exceptions are seen for the following compounds: (a) (T(tBu)P)Ni (5), which is very nonplanar and has a porphyrin π-cation radical with mixed a1u/a2u character);68 (b) (T(C3F7)P)Ni (6), which likely has an a1u porphyrin π-cation radical as evidenced by a Q(0,0) band that is more intense than the Q(1,0) band60 and by the fact that the electron-withdrawing C3F7 substituents at the meso positions will preferentially stabilize the a2u orbital; and (c) (F20TPP)Ni (24), which is assigned as an a1u radical because perfluorination of the phenyl ring switches the radical from a2u in (TPP)Zn to a1u in (F20TPP)Zn.69 All the investigated T(X)OEP and Br8T(aryl)P derivatives (see Table 1) are likely to form a1u radicals upon oxidation,32,67,70 with the exception of (Br8TPP)Ni (17), which is known to form an a2u π-cation radical.61 As shown in Table 4, the radical assignments proposed for the π-cations of the D(aryl)Ps compounds are dependent upon the substituents. [(DPP)Ni]+ (19) has been assigned as a2u on the basis of its ESR spectrum.47 As shown in Table 3, the π-cation radicals of (F4DPP)Ni and (F8DPP)Ni have ESR features virtually identical to that of [(DPP)Ni]+, thus indicating that all three π-cation radicals are a2u. In addition, the UV-visible spectra of [(DPP)Ni]+, [(F4DPP)Ni]+, [(F8DPP)Ni]+, and [(F12DPP)Ni]+ present the same features, thus suggesting that the π-cation radical of (F12DPP)Ni is also a2u. Those derivatives without fluorine groups at the ortho positions of the meso phenyl rings, i.e., [DPP(Ni)]+, [(F4DPP)Ni]+, [(F8DPP)Ni]+, and [(F12DPP)Ni]+ are thus assigned as a2u radicals. The radical types of the remaining D(Ar)Ps were assigned on the basis of the electron-withdrawing ability of the aryl substituents and the optical spectra of the oxidized species. [(F8DPP(meso))Ni]+, [(F20DPP)Ni]+, [(F28DPP)Ni]+, and [(F36DPP)Ni]+ were assigned as a1u based on the presence of the very electron-withdrawing aryl groups and a stronger (64) Czernuszewicz, R. S.; Macor, K. A.; Li, X.-Y.; Kincaid, J.; Spiro, T. G. J. Am. Chem. Soc. 1989, 111, 3860. (65) Fajer, D.; Davis, M. S. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1979; Vol. IV, p 197. (66) Spellane, P. J.; Gouterman, M.; Antipas, A.; Kim, S.; Liu, Y. C. Inorg. Chem. 1980, 19, 386. (67) Sibilia, S. A.; Hu, S.; Piffat, C.; Melamed, D.; Spiro, T. G. Inorg. Chem. 1997, 36, 1013. (68) Lin, C.-Y.; Hu, S.; Rush, T. R. I.; Spiro, T. G. J. Am. Chem. Soc. 1996, 118, 9452. (69) Yang, S. I.; Seth, J.; Strachan, J.-P.; Gentemann, S.; Kim, D.; Holten, D.; Lindsey, J. S.; Bocian, D. F. J. Porphyrins Phthalocyanines 1999, 3, 117. (70) Piffat, C.; Melamed, D.; Spiro, T. G. J. Phys. Chem. 1993, 97, 7441.

Q(0,0) band than Q(1,0) band.60 As shown in Table 3, this feature is also observed in the UV-visible spectra of (F8DPP(meso))Ni, (F20DPP)Ni, (F28DPP)Ni, and (F36DPP)Ni, supporting the idea that all four derivatives possess a1u HOMOs and form a1u radicals. Based on the electronic characteristics of the porphyrin substituents, [(T(4-NO2-P)OPP)Ni]+ was tentatively assigned as an a1u radical and [((OMe)20DPP)Ni]+ and [(OPT(3-Th)P)Ni]+ as a2u radicals. The relationships between ∆|Ox2 - Ox1| and Red(1) for different groups of porphyrins in dichloromethane solutions containing 0.1 M TBAP or 0.1 M TBAPF6 are shown in Figure 6. Nickel porphyrins with nonplanar macrocycles (i.e., saddle, ruffle, or mixed) and a2u type π-cation radicals, nominally planar porphyrins with a2u type π-cation radicals, and nonplanar macrocycles with a1u type π-cation radicals are shown in separate plots. Insufficient data were available to produce a plot for planar porphyrins with a1u type π-cation radicals. The plots with TBAP as the supporting electrolyte consist of two regions: one where the two oxidations of the nickel porphyrins are overlapped (∆|Ox2 - Ox1| ) 0) independent of the potentials at which the compounds are reduced, and another where the values of ∆|Ox2 - Ox1| increase linearly as E1/2 becomes more negative. This general pattern is again consistent with a model in which binding of the supporting electrolyte to the dication is important in determining ∆|Ox2 - Ox1|. In the region where the two oxidations of the nickel porphyrins are overlapped, the substituents are very electron withdrawing and binding of the anion to the dication is strong, which lowers the potential for the second oxidation. In the region where the value of ∆|Ox2 - Ox1| increases linearly as E1/2 becomes more negative, the substituents are less electron withdrawing and anion binding is correspondingly weaker. In this region, ∆|Ox2 - Ox1| is dependent upon the electron-withdrawing ability of the substituents (as measured by E1/2 for macrocycle reduction), which determines the anion binding affinity. The plots obtained with TBAPF6 as the supporting electrolyte contain fewer compounds in the region where the two oxidations of the nickel porphyrins are overlapped, consistent with the known tendency of PF6- anions to bind less strongly.62,63 Correlation coefficients, intercepts, and slopes for the regions where ∆|Ox2 - Ox1| increases linearly are summarized in Table S5 of the Supporting Information, and the intercepts are also indicated by an arrow in Figure 6. The correlation coefficients are sometimes poor, although the differences between the plots are consistent. Specifically, independent of the supporting electrolyte (and of whether a compound is included on the portion of the graph where ∆|Ox2 - Ox1| ) 0), the intercept of the two lines at ∆|Ox2 - Ox1| ) 0 occurs at a more positive reduction potential in the order nonplanar a1u radicals, planar a2u radicals, and nonplanar a2u radicals. In other words, the plots indicate that independent of the supporting electrolyte used in the electrochemical measurements, more positive reduction potentials (i.e., more electron-withdrawing substituents) are required to overlap the two oxidations for a1u vs a2u type radicals of nonplanar porphyrins and for planar vs nonplanar Inorganic Chemistry, Vol. 41, No. 25, 2002

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Figure 6. Plots of ∆|Ox2 - Ox1| versus E1/2 for reduction in dichloromethane containing 0.1 M TBAP or 0.1 M TBAPF6 for nickel porphyrins with (a) nonplanar a2u radicals, (b) planar a2u radicals, and (c) nonplanar a1u radicals. Arrows indicate intercepts of the best regression line with the line at which ∆|Ox2 - Ox1| ) 0.

macrocycles of a2u type radicals. Furthermore, for the same group of compounds (e.g., nonplanar a1u radicals), more positive intercepts at ∆|Ox2 - Ox1| are seen for TBAPF6 versus TBAP (Figure 6), consistent with weaker complexation of the perchlorate anion requiring more electronwithdrawing substituents to cause overlap of the oxidation potentials. The slopes of the regions where ∆|Ox2 - Ox1| increases linearly as E1/2 becomes more negative are also clearly steeper for the nonplanar a2u radicals versus the nonplanar a1u radicals. What is the significance of the dependence of the intercept on the type of radical and the porphyrin conformation, and how can these trends be explained in terms of the proposed anion binding model? Crystallographic data establish that the anions in porphyrin π-cation radicals typically bind in the vicinity of the metal atom in both nominally planar porphyrins and in very nonplanar porphyrins. For example, the distance between the metal atom and an oxygen atom of the perchlorate anion is 2.079(8) Å in [(TPP)Zn]+(ClO4), 2.012(5) Å in [(TPP)Mg]+(ClO4), and 2.13(1) Å in [(TPP)FeIII]2+(ClO4)2.24 In [(OETPP)Cu]+(ClO4), the copperoxygen distance is 2.445(4) Å.71 Assuming that the binding


site of the anions is similar for the examined Ni(II) dications, an a2u versus a1u type radical or nonplanar versus planar conformation may generate increased positive charge on the nickel atom or allow better delocalization of any negative charge transferred from the anion. This then favors tighter anion binding and overlap of the oxidations at more negative reduction potentials (i.e., with less electron-withdrawing substituents). It should be pointed out that if the HOMO is an a2u orbital, then the HOMO-1 is an a1u orbital, and that when two electrons are abstracted from the HOMO (to form a dication), the HOMO of the dication has a profile opposite that of the original HOMO; i.e., an a2u cation radical will result in an a1u dication and an a1u cation radical will result in an a2u dication. With this in mind, it is known that a2u π-cation radicals have the unpaired electron spin (or the equivalent positive hole) delocalized on the meso carbon atoms and the nitrogen atoms, whereas a1u π-cation radicals have the unpaired electron spin (or positive hole) delocalized on the (71) Renner, M. W.; Barkigia, K. M.; Zhang, Y.; Medforth, C. J.; Smith, K. M.; Fajer, J. J. Am. Chem. Soc. 1994, 116, 8582.


R and β carbon atoms of the pyrrole rings.72 Therefore, more positive potentials for overlap of the oxidations (i.e., lower anion binding affinities) may be seen for a1u radicals because the a2u dication derived from this radical has a filled p orbital on the nitrogen atoms and there is thus a greater π-electron density near the nickel atom. This in turn would make it more difficult to bind the anion and/or delocalize any charge transferred from the anion, thus requiring more electronwithdrawing substituents to overlap the oxidations. The plots in Figure 6 also show that the intercepts at ∆|Ox2 - Ox1| ) 0 shift to more negative potentials for nonplanar a2u radicals compared to planar a2u radicals, suggesting that anion binding is stronger for the π-dications of nonplanar porphyrins. This effect appears to be weaker than the effect arising from the different types of radicals discussed above. Previous studies have shown that nonplanarity typically decreases axial amine ligand affinity,14 in part because the substituents resist the core expansion that has to occur when the nickel atom binds axial ligands and becomes high spin. Why then does nonplanarity shift the intercepts at ∆|Ox2 Ox1| ) 0 to more negative potentials indicative of higher anion binding affinities? One possibility is an increased interaction between the substituents and the metal center in nonplanar porphyrins versus planar porphyrins which leads to stronger anion binding in the former. In other words, the substituent effect is transmitted more strongly to the probable anion binding site (the nickel atom) when the macrocycle is nonplanar. Note that there is some precedent in the literature for such an effect; it has been shown that nonplanarity influences the interaction between the unpaired electrons on the macrocycle and the metal atom in the copper(II) complex of OETPP (13) by permitting the overlap of orbitals that are orthogonal in a planar porphyrin system.71 Overall, the plots in Figure 6 appear to be fully consistent with the idea that the dominant effect determining the gap between the macrocycle oxidation potentials in the nickel porphyrins is stabilization of the π-dication by binding of the supporting electrolyte. The magnitude of ∆|Ox2 - Ox1| is apparently linked to the complexing ability of the anion (which is larger for perchlorate than for hexafluorophosphate)62,63 and the electron-withdrawing ability of the substituents (i.e., the electron density at the anion binding site). The present work also suggests that the anion binding affinity depends on other features of the macrocycle such as the type of π-cation radical and the conformation of the neutral molecule. Of note is the fact that the plots in Figure 6 still show some residual scatter even when the type of radical and the macrocycle conformation are considered. This scatter may originate from several sources such as different amounts and types of nonplanar deformation in the nonplanar porphyrins, the π-dications not possessing pure a1u or a2u characteristics, or electrostatic interactions between the anions and the porphyrin substituents which also affect anion binding. In summary, the present study addresses in more detail the question of which structural and experimental features

encourage overlap of the macrocycle oxidations in nickel porphyrins. Factors which are found to favor overlap of the oxidations include (a) the presence of very electronwithdrawing substituents on the porphyrin macrocycle, (b) the use of a weakly polar solvent and a strongly coordinating anion in the supporting electrolyte,73 and (c) the formation of an a2u type radical rather than an a1u type radical. Nonplanar deformations of the porphyrin macrocycle also appear to favor overlap of the oxidations, although this effect is not particularly strong. Studies are currently in progress to determine if the mechanism suggested by the nickel porphyrin data also operates for other metalloporphyrins. Electrochemical data from the literature suggest that this is indeed the case.74

(72) Gouterman, M. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. 3, pp 1-165.

(73) Geng, L.; Murray, R. W. Inorg. Chem. 1986, 25, 3115. (74) Woller, E. K.; DiMagno, S. G. J. Org. Chem. 1997, 62, 1588.

Experimental Section Electrochemistry. Ultra-high-purity N2 from Trigas was used to deoxygenate all solutions prior to each electrochemical experiment. Absolute dichloromethane (CH2Cl2) was purchased from Fluka Chemical Co. and purified as follows: It was first washed with sulfuric acid several times and stirred overnight until the yellowish color of the sulfuric acid phase disappeared. It was then washed with distilled water and dried with magnesium sulfate before being doubly distilled, first over phosphorus pentoxide (P2O5) and then over calcium hydride. TBAPF6 and TBAP were purchased from Fluka Chemical Co. Both supporting electrolytes were recrystalized twice from absolute ethyl alcohol (TBAP) or ethyl acetate (TBAPF6) and then dried in a vacuum oven at 40 °C for 3 days prior to use. Cyclic voltammograms were obtained with an EG&G Model 173 potentiostat or 263 A potentiostat or an IBM Model EC 225 voltammetric analyzer. Current-voltage curves were recorded on an EG&G Princeton Applied Research Model RE-0151 X-Y recorder or a Zenith data system Model Z-386 SX/20 computer coupled with a Hewlett-Packard deskjet 600 plotter. A threeelectrode system was used and consisted of a glassy carbon or a platinum button working electrode, a platinum wire counter electrode, and a saturated calomel reference electrode (SCE). This reference electrode was separated from the bulk solution by a frittedglass bridge filled with the solvent/supporting electrolyte mixture. All potentials are referenced to the SCE. X-ray Crystallography. X-ray diffraction data were collected with a Syntex P21 diffractometer with a sealed tube source [λ(Cu KR) ) 1.541 78 Å] and a locally modified LT-1 low-temperature apparatus for 6, a Bruker SMART 1000 diffractometer with a sealed tube source [λ(Mo KR) ) 0.710 73 Å] and a Cryoindustries lowtemperature apparatus for 11, and a Siemens P3 diffractometer with a sealed tube source [λ(Mo KR) ) 0.710 73 Å] and a locally modified Nonius low-temperature apparatus for 15. The Bruker SHELXTL V. 5.03 software package was used for structure solution and refinement; scattering factors were used as supplied. Structures were refined based on F2 using all independent data by full matrix least-squares methods. Illustrations of the crystal structures of compounds 6, 11, and 15 are provided in Figure 3. A summary of experimental details is given in Table 6. Full experimental details, in CIF format, are available as Supporting Information to this article. Molecular Mechanics Calculations. Molecular mechanics calculations were carried out using POLYGRAF software (Molec-


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Kadish et al. ular Simulations, Inc.) and a force-field for metalloporphyrins that has been described in the literature.14,15 Synthesis. Electrochemical data for porphyrins 1-3 and 8-10 was taken from the literature.10 (F20TPP)Ni (4),75 (T(tBu)P)Ni (5),21 (T(iPr)P)Ni (7),21 (OETPP)Ni (13),14,35 (OETNP)Ni (14),38 (TC6TPP)Ni (16),14,76 (OET(3-Th)P)Ni (12),77 (Br8TPP)Ni (17),78 (Br8F20TPP)Ni (18),79 (DPP)Ni (19),40 and (OPT(3-Th)P)Ni (24)77 were prepared according to literature procedures. The remaining nickel porphyrins were prepared as described in the following sections. The nickel complexes were prepared by inserting nickel into the corresponding free base porphyrins using nickel acetylacetonate in refluxing xylenes or toluene. The nickel porphyrins were typically purified using column chromatography and recrystallization. 1H and 19F NMR spectra were measured at frequencies of 300 and 283 MHz, respectively. Spectra were typically recorded at ambient temperature (298 ( 5 K) using 2-5 mM solutions. 1H chemical shifts were referenced to TMS, CHCl3 at δ 7.26, C6D5CHD2 at δ 2.09, or CD3COCHD2 at δ 2.05. 19F chemical shifts were referenced to internal CF2Cl2 at -8.0 ppm.80 Visible absorption spectra were recorded on a Hewlett-Packard 8450A spectrophotometer using CH2Cl2 as solvent. High-resolution FAB or EI mass spectra were obtained from the University of California, Riverside, facility. Lowresolution MALDI spectra were obtained at UC Davis as described previously.81 Melting points (uncorrected) were measured on a Thomas/Bristoline microscopic hot stage apparatus. 5,10,15,20-Tetrakis(heptafluoropropyl)porphinatonickel(II), (T(C3F7)P)Ni (6). The free base porphyrin (T(C3F7)P)H2 was prepared as described previously28,29 and nickel inserted in 80% yield using the nickel acetylacetonate/toluene procedure. mp >300 °C. HRMS Calcd for C32H8F28N4Ni: 1037.966; found: 1037.965. 1H NMR (CDCl3): 9.32 (s, 8 H, β-H). 19F NMR (CD2Cl2): -120.0 (s, 8 F, CF2CF2CF3), -86.7 (br, 8 F, CF2CF2CF3), -81.2 (s, 12 F, CF3). Visible spectrum (CH2Cl2), λ/nm (/cm-1 mol-1 dm-3): 406 (95.4), 548 (6200), 588 (17 200). Anal. Calcd for C32H8 F28N4Ni: C, 36.99; H, 0.78; N, 5.39. Found: C, 37.21, H, 0.79, N, 5.47. 5,10,15,20-Tetrabenzoate-2,3,7,8,12,13,17,18-octaethylporphinatonickel(II), (T(Bz)OEP)Ni (11). (OEP)Ni (300 mg, 0.503 mmol) was dissolved in dichloromethane (100 mL), benzoyl peroxide (0.6 g, 2.48 mmol) was added, and the mixture was stirred for 3 days at room temperature. The reaction mixture was washed with sodium carbonate and then with water, and the solvent was removed under reduced pressure. The product was purified by silica gel column chromatography using dichlromethane/cyclohexane as eluent, and recrystallized from dichlromethane/cyclohexane to afford the title compound in 45.8% yield. 1H NMR (C6D5CD3): 8.47 (m, 8H, Hortho), 7.22 (m, 12H, Hmeta and Hpara), 3.40 and 3.61 (m, 8H each, CH2 A and B), 1.57 (t, 24H, CH3). Visible spectrum (CH2Cl2), λ/nm (/cm-1 mol-1 dm-3): 424 (289 000), 543 (22 000), 583 (7000). (75) Carvalho de Medeiros, J. A.; Cosnier, S.; Deronzier, A.; Moutet, J.C. Inorg. Chem. 1996, 35, 2659. (76) Medforth, C. J.; Berber, M. D.; Smith, K. M.; Shelnutt, J. A. Tetrahedron Lett. 1990, 31, 3719. (77) Medforth, C. J.; Haddad, R. E.; Muzzi, C. M.; Dooley, N. R.; Jaquinod, L.; Shyr, D. C.; Nurco, D. J.; Olmstead, M. M.; Smith, K. M.; Ma, J.-G.; Shelnutt, J. A., submitted for publication. (78) Bhyrappa, P.; Krishnan, V. Inorg. Chem. 1991, 30, 239. (79) Mandon, D.; Ochsenbein, P.; Fischer, J.; Weiss, R.; Jayaraj, K.; Austin, R. N.; Gold, A.; White, P. S.; Brigaud, O.; Battioni, P.; Mansuy, D. Inorg. Chem. 1992, 31, 2044. (80) Harris, R. K.; Mann, B. E. NMR and the Periodic Table; Academic Press: New York, 1978. (81) Green, M. K.; Medforth, C. J.; Muzzi, C. M.; Nurco, D. J.; Shea, K. M.; Smith, K. M.; Shelnutt, J. A.; Lebrilla, C. B. Eur. Mass Spectrom. 1997, 3, 439.


2,3,7,8,12,13,17,18-Octaethyl-5,10,15,20-tetrakis(pentafluorophenyl)porphinatonickel(II), (F20OETPP)Ni (15). The free base porphyrin (F20OETPP)H2 was prepared in 10.1% yield using the procedure employed in the synthesis of (OETPP)H2.35 1H NMR (CDCl3): 3.80 and 2.18 (br, 8H each, CH2 A and B), 0.68 (br, 24H, CH3), -1.7 (v br, 2H, NH). 19F NMR (CDCl3): -137.7 (Fortho), -153.0 (Fpara), -163.2 (Fmeta). Nickel was inserted using the nickel acetylacetonate/xylene procedure. 1H NMR (CDCl3): 2.42 (br, 16H, CH2), 0.70 (t, 24H, CH3). 19F NMR (CDCl3): -137.6 (Fortho), -153.2 (Fpara), -163.1 (Fmeta). Visible spectrum (CH2Cl2), λ/nm (/relative intensity): 426 (1.00), 558 (0.058), 595 (0.124). 5,10,15,20-Tetrakis(4-fluorophenyl)-2,3,7,8,12,13,17,18-octaphenylporphinatonickel(II), (F4DPP)Ni (20). The free base porphyrin was prepared as described previously82 and nickel inserted in 83% yield using the nickel acetylacetonate/xylene procedure. mp >300 °C. 1H NMR (CDCl3), δ (ppm): 6.79 (t, 8H, β-Hpara), 6.78 (t, 8H, meso-Hortho), 6.69 (t, 16H, β-Hmeta), 6.59 (d, 16H, β-Hortho), 6.07 (t, 8H, meso-Hmeta). 19F NMR (CDCl3): -118.8. FAB HRMS [M]+ calcd 1350.3795, found 1350.3857. Visible spectrum (CH2Cl2), λ/nm (/cm-1 mol-1 dm-3): 446 (213 000), 564 (16 200), 608 (16 700). Anal. Calcd for C92H56F4N4Ni‚CH3OH: C, 80.70; H, 4.37; N, 4.05. Found: C, 80.86; H, 4.27; N, 4.20. 2,3,7,8,12,13,17,18-Octakis(4-fluorophenyl)-5,10,15,20-tetraphenylporphinatonickel(II), (F8DPP)Ni (21). The free base porphyrin was prepared as described previously82 and nickel inserted in 75% yield using the nickel acetylacetonate/xylene procedure. mp >300 °C. FAB HRMS [M]+ calcd 1422.3418, found 1422.3374. 1H NMR (CDCl ), δ (ppm): 6.87 (d, 8H, meso-H 3 ortho), 6.80 (t, 4H, meso-Hpara), 6.54 (t, 8H, meso-Hmeta), 6.52 (t, 16H, β-Hortho), 6.35 (t, 16H, β-Hmeta). 19F NMR (CDCl3): -119.1. Visible spectrum (CH2Cl2), λ/nm (/cm-1 mol-1 dm-3): 446 (200 000), 566 (15 500), 610 (15 600). Anal. Calcd for C92H52F8N4Ni‚CH3OH: C, 76.77; H, 3.88; N, 3.85. Found: C, 77.10; H, 4.18; N, 3.78. 2,3,5,7,8,10,12,13,15,17,18,20-Dodecakis(4-fluorophenyl)porphinatonickel( II), (F12DPP)Ni (22). The free base porphyrin was prepared as described previously82 and nickel inserted in 75% yield using the nickel acetylacetonate/xylene procedure. mp >300 °C. FAB HRMS [M]+ calcd 1494.3041, found 1494.3086. 1H NMR (CDCl3): 6.82 (q, 8H, meso-Hortho), 6.54 (m, 16H, β-Hortho), 6.45 (t, 16H, β-Hmeta), 6.26 (t, 8H, meso-Hmeta). 19F NMR (CDCl3): -117.9 (β-Fpara), -116.9 (meso-Fpara). Visible spectrum (CH2Cl2), λ/nm (/cm-1 mol-1 dm-3): 446 (206 000), 564 (15 500), 608 (15 700). Anal. Calcd for C92H48N4F12Ni‚2(MeOH): C, 72.37; H, 3.62; N, 3.59. Found: C, 72.59; H, 3.47; N, 3.61. 2,3,7,8,12,13,17,18-Octakis(4-methoxyphenyl)-5,10,15,20-tetrakis(3,4,5-trimethoxyphenyl)porphinatonickel(II), ((OMe)20DPP)Ni (23). 3,4,5-Trimethoxybenzaldehyde (694 mg, 3.54 mmol) was dissolved in refluxing acetic acid (33 mL) and 3,4-bis(4-methoxyphenyl)pyrrole (988 mg, 3.54 mmol) dissolved in warm acetic acid (20 mL) was added to the refluxing solution. Reflux was continued for 17, h after which time DDQ (803 mg, 3.54 mmol) was added to the reaction mixture and reflux continued for another 2 h. The reaction mixture was poured into deionized water (500 mL) and extracted with dichloromethane (4 × 150 mL). The combined organic extracts were washed with deionized water (400 mL). The organic phase was shaken vigorously with aqueous 2% NaOH to convert the product to its free base form. A silica gel column (20 cm × 5 cm packed as a slurry in CH2Cl2) was eluted with a gradient of MeOH/CH2Cl2 (starting with 1% and finishing (82) Kadish, K. M.; Van Caemelbecke, E.; D’Souza, F.; Lin, M.; Nurco, D. J.; Medforth, C. J.; Forsyth, T. P.; Krattinger, B.; Smith, K. M.; Fukuzumi, S.; Nakanishi, I.; Shelnutt, J. A. Inorg. Chem. 1999, 38, 2188.

OxidatiWe BehaWior of Nickel Porphyrins with 40% MeOH). Though free base material was loaded on the column, primarily dication was recovered in the eluate. After a basic aqueous washing and crystallization, purified ((OMe20)DPP)H2 was obtained in 58% yield (932 mg, 0.511 mmol). mp >300 °C. FAB HRMS [MH]+ calcd for C112H103N4O20 1823.7166, found 1826.7240; MALDI FT-ICR MS [MH]+ 1823.7. 1H NMR (CDCl3): 3.63 (s, 24H, methoxyl), 3.76 (s, 12H, methoxyl), 3.79 (s, 24H, methoxyl), 6.33, 6.69 (doublets, 16H each, β-Hortho and β-Hmeta), 6.85 (s, 8H, meso-Hortho). Visible spectrum (CH2Cl2), λ/nm (/relative intensity): 472 (100), 562 (7.2), 612 (6.2), 718 (4.1). Visible spectrum (CH2Cl2 plus 1% trifluoroacetic acid), λ/nm (/relative intensity): 506 (100), 726 (26.4). The nickel complex was prepared in 91% yield using the nickel acetylacetonate/xylene procedure. mp >300 °C. FAB HRMS [M]+ calcd for C112H100N4O20Ni 1878.6284, found 1878.6211; MALDI FT-ICR MS [M]+ 1878.6. 1H NMR (CDCl3): 6.60, 6.31 (d, 16H each, β-Hmeta and β-Hortho), 6.22 (s, 8H, mesoHortho), 3.69 (s, 12H, meso-OMepara), 3.63, 3.53 (s, 24H each, mesoOMemeta and β-OMe). Visible spectrum (CH2Cl2), λ/nm (/cm-1 mol-1 dm-3): 452 (138 000), 568 (12 000), 612 (15 100). 5,10,15,20-Tetrakis(2,6-difluorophenyl)-2,3,7,8,12,13,17,18-octaphenylporphinatonickel(II), (F8DPP)Ni (meso) (25). The free base porphyrin was prepared as described previously82 and nickel inserted in 57% yield using the nickel acetylacetonate/xylene procedure. mp >300 °C. FAB HRMS [M]+ calcd for C92H52N4F8Ni 1422.3418, found 1422.3349. 1H NMR (CDCl3): 6.73 (m, 40H, β-phenyl), 6.51 (m, 4H, meso-Hpara), 5.92 (t, 8H, meso-Hmeta). 19F NMR (CDCl3): -108.4. Visible spectrum (CH2Cl2), λ/nm (/cm-1 mol-1 dm-3): 436 (149 000), 558 (12 100), 598 (14 500). 5,10,15,20-Tetrakis(pentafluorophenyl)-2,3,7,8,12,13,17,18-octaphenylporphinatonickel(II), (F20DPP)Ni (26). The free base porphyrin was prepared as described previously82 and nickel inserted in 86% yield using the nickel acetylacetonate/xylene procedure. mp >300 °C. FAB HRMS [M]+ calcd 1638.2287, found 1638.2270. 1H NMR (CDCl ): 6.95 (m, 40H, β-phenyl). 19F NMR (CDCl ): 3 3 -166.4 (Fmeta), -156.6 (Fpara), -136.4 (Fortho). Anal. Calcd for C92H40F20N4Ni: C, 67.38; H, 2.46; N, 3.42. Found: C, 67.04; H, 2.49; N, 3.35. Visible spectrum (CH2Cl2), λ/nm (/cm-1 mol-1 dm-3): 432 (211 000), 556 (24 700), 596 (33 800). 2,3,7,8,12,13,17,18-Octakis(4-fluorophenyl)-5,10,15,20-tetrakis(pentafluorophenyl)porphinatonickel(II), (F28DPP)Ni (27). The free base porphyrin was prepared as described previously82 and nickel inserted in 98% yield using the nickel acetylacetonate/ xylene procedure. mp >300 °C. FAB HRMS [M]+ calcd 1782.1533, found 1782.1506. 1H NMR (CDCl3): 6.85 (m, 16H, β-Hortho) 6.70 (t, 16H, β-Hmeta). 19F NMR (CDCl3): -165.6 (Fmeta), -154.9 (mesoFpara), -136.2 (Fortho), -115.0 (β-Fpara). Anal. Calcd for C92H32F28N4-

Ni: C, 61.94; H, 1.81; N, 3.14. Found: C, 61.94; H, 1.90; N, 3.14. Visible spectrum (CH2Cl2), λ/nm (/cm-1 mol-1 dm-3): 432 (186 000), 556 (15 100), 596 (23 200). 2,3,7,8,12,13,17,18-Octakis(3,5-difluorophenyl)-5,10,15,20-tetrakis(pentafluorophenyl)porphinatonickel(II), (F36DPP)Ni (28). The free base porphyrin was prepared as described previously82 and nickel inserted in 81% yield using the nickel acetylacetonate/ xylene procedure. mp >300 °C. FAB HRMS [M]+ calcd for C92H24N4F36Ni 1926.0780, found 1926.0710. 1H NMR (CD3COCD3): 6.47 (m, 16H, β-Hortho), 6.63 (m, 8H, β-Hpara). 19F NMR (CDCl3): -164.4 (meso-Fmeta), -152.6 (Fpara), -135.7 (Fortho), -110.6 (β-Fmeta). Visible spectrum (CH2Cl2), λ/nm (/relative intensity): 434 (1.00), 557 (0.086), 595 (0.109). 5,10,15,20-Tetrakis(4-nitrophenyl)-2,3,7,8,12,13,17,18-octaphenylporphinatonickel(II), (T(4-NO2-P)OPP)Ni (29). The free base porphyrin was prepared using a previously published procedure.83,84 Nickel was inserted using the nickel acetylacetonate/xylene procedure. 1H NMR (CDCl3): 7.67 and 7.55 (d, 8H each, mesoHortho and meso-Hmeta), 6.80 (m, 8H, β-Hpara), 6.66 (t, 16H, β-Hmeta), 6.59 (m, 16H, β-Hortho). Visible spectrum (CH2Cl2), λ/nm (/cm-1 mol-1 dm-3): 450 (216 000), 566 (21 700), 612 (23 100).

Acknowledgment. The support of the Robert A. Welch Foundation (K.M.K., Grant E-680) is gratefully acknowledged. This work was supported by grants from the National Institutes of Health (HL 22252) and the National Science Foundation (CHE-99-04076) (K.M.S.). This work was partially supported by the Division of Material Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed-Martin company, for the United States Department of Energy under Contract No. DEACO4-94AL85000. Supporting Information Available: Molecular mechanics energies for (D(Ar)P)Ni complexes, Hammett σ parameters and ∑σ values, and analyses of the plots of E1/2 for oxidation of reduction versus ∑σ and ∆|Ox2 - Ox1| versus E1/2 for macrocycle reduction. Details of the crystal structure determinations of compounds 6, 11, and 15 (CIF). This material is available free of charge via the Internet at http://pubs.acs.org. IC0200702 (83) (84) (85) (86)

Takeda, J.; Sato, M. Inorg. Chem. 1992, 31, 2877. Takeda, J.; Sato, M. Chem. Pharm. Bull. 1994, 42, 1005. Cullen, D. L.; Meyer, E. F. J. Am. Chem. Soc. 1974, 96, 2095. Meyer, E. F. Acta Crystallogr., Sect. B 1972, B28, 2162.


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Influence of Electronic and Structural Effects on the Oxidative Behavior

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